Conventional scintillators have been developed for detection of high energy particles and radiation, such as x-rays, gamma-rays, neutrons, and the like. A scintillation detecting system is based on the use of a scintillation composition to convert a portion of the energy imparted to the composition by incident ionizing radiation, to light, such as visible or ultra-violet scintillation light. Absolute scintillation (or conversion) efficiency of a composition is defined as the ratio of the energy carried by the visible or ultra-violet light, to the energy lost in the composition by the incident ionizing radiation. The light emerging from a scintillator typically impinges upon some photo-electric device, e.g., a photomultiplier (PM), or charge coupled device (CCD), where it is converted into an electrical pulse. This electrical pulse is then amplified and recorded by a standard electronic data acquisition system. Details of scintillators, in general, and plastic organic scintillators, in particular, are described in publications such as the books by J. B. Birks, “The Theory and Practice of Scintillation Counting”, Pergamon Press, (1964), and by G. F. Knoll, “Radiation Detection and Measurement”, J. Wiley and Sons 1989 particularly Chapter 8. Plastic scintillators may be a solid sheet or plate or may be in the form of an optical fiber or fiber optic plate such as disclosed in European Patent Publication 0 606 732 A1, Jul. 20, 1994.
Conventional plastic scintillators (ternary scintillator) typically have three components, such as a polymeric matrix, e.g., poly(vinyltoluene) (PVT), and two fluors (fluorescent compounds). The typical scintillator composition of the two fluors is a primary dye, e.g., para-terphenyl (PTP), and a secondary dye, e.g., diphenylstilbene (DPS), at concentrations of about 1% and 0.02% wt./wt., respectively. Such a scintillator material is haze free, optically transparent, solid and stable. Methods of making and using such conventional plastic scintillators are disclosed in Harrah et al., U.S. Pat. No. 4,594,179. It has been observed that the light output from the conventional scintillator does not increase as the PTP concentration is increased above 1% wt./wt. This phenomenon has been described as “concentration quenching”, and is caused by several underlying physical mechanisms.
Generally, a high absolute scintillation efficiency of a scintillator composition is desirable to achieve high detection sensitivity of ionizing radiation. Scintillation efficiency is a function of several parameters, including the type of solid matrix and the type of fluors employed. Typically, light output relative to anthracene is less than 70% for plastic and the absolute scintillation efficiencies is less than about 4%. Since modern scintillator solute fluors typically have fluorescent quantum efficiencies of close to 100%, a substantial increase in plastic composition scintillation efficiency by alternative choices of fluors is unlikely. For this reason, the light output from commercial plastic scintillator has remained at less than 70% of the light output from anthracene for more than 60 years.
Attempts have been made to increase scintillation efficiency of plastic scintillators by using other plastic matrixes such as polyvinylxylene, polyisopropyl styrene, and polyvinyl naphthalene, and copolymers of monomers represented in polymers listed above. Such attempts have resulted in increasing the scintillation efficiency by up to about 40%. Such approaches suffer from one or more disadvantages, such as the monomers or polymers are commercially unavailable or prohibitively expensive, or polymer compositions are brittle and subject to surface crazing or deterioration. For these reasons, none of these approaches has been pursued commercially.
Addition of naphthalene to conventional plastic scintillators has been explored as a way to increase the scintillation efficiency. Brown, et al. (Nuclear Electronics 1, 15, 1959) added naphthalene to solid plastic scintillators, where polystyrene (PS) and polymethylmethacrylate (PMMA) were used as matrices. Addition of less than about 3% by weight of naphthalene to a PS mixture containing the fluor 2,5-diphenyl oxazole (PPO), did not change maximum scintillation efficiency of the mixture. When about 10% by weight of naphthalene is added to PMMA, this polymer is transformed from an extremely inefficient matrix to one with about 50% of the scintillation efficiency of PS.
J. Tymianski and J. K. Walker, U.S. Pat. No. 5,606,638, used polystyrene with 15% by weight of the following fluorescent aromatic compounds: dimethylnaphthalene, acenaphthene, and fluorene. In each case a fluorescent dye, tetraphenylbutadiene (TPB), was added at 1% weight. The purpose of the TPB was to absorb energy from the excited aromatic compound and from polystyrene and provide subsequent emission of scintillation light at about 420 nm. The relative scintillation emission output of these scintillating compositions compared to a composition containing only TPB were found to be as follows: Dimethylnaphthalene 1.51; Acenaphthene 1.49; and Fluorene 1.47. In each scintillator, there is a substantial and almost equal increase in scintillating light emission. Taking into account the fact that the quantum yields of the three aromatic compounds are 0.22, 0.6, and 0.8, respectively, it suggests that there is severe self-quenching of these dyes especially in the latter two cases.
Although many efforts have been made to produce more efficient plastic scintillator material, there still exists a need to produce plastic scintillator with light output relative to anthracene of at least 125%, preferably greater than 150%, and most preferably greater than 175%.